Heretofore, tripod heads on stands or tripods have been frequently used to enable illumination devices, various cameras, measurement instruments, and so on to be mounted at an adjustable angle of depression or elevation.
One example of these tripod heads which are adapted for use with a relatively small-sized, lightweight device to be held such as an illumination device is described, for example, in patent reference 1 (JP-UM-B-5-29462) that a support base is supported on a stand, a mounting member for mounting a tubular base member mounted in the base of an illumination device is supported directly to the support base tiltably with a threaded rod facing in the horizontal direction and thus, the illumination device can be mounted tiltably. However, where the illumination device is large in size and relatively heavy, sufficient holding force is not obtained only with the threaded rod. Therefore, a more rugged tilt angle-adjusting mechanism is required.
A tripod head coping with this problem is already known and shown in FIG. 11. This tripod head has a mounting member 53 for mounting the base portion of an illumination device or the like to a support base 52, and when the mounting member 53 is tightened and fastened to the support base 52, the mounting member 53 is tiltably pivoted to the support base 52 via a mounting shaft 54. An incomplete disk 53b coaxial with the mounting shaft 54 is mounted integrally with the mounting member 53. An arc-shaped groove 55 is formed in a peripheral portion of the disk 53b and located around the axis of the mounting shaft 54. A threaded rod 56 standing upright from the support base 52 is inserted in the arc-shaped groove 55. A nut 57 equipped with a handle tightens the fringes of the arc-shaped groove 55 against the support base 52 via a frictional member 58. The body portion 53a of the mounting member 53 for mounting the device to be held such as an illumination device is integrally mounted to the disk 53b. 
In FIG. 11, reference numeral 59 denotes securing hole which is provided in the body portion 53a of the mounting member 53 and into which the tubular base member provided to the base portion of the illumination device or the like is inserted and secured. Reference numeral 60 denotes a nut with a handle to tighten the hole 59 therewith.
With the tripod head of this construction, the mounting member 53 for the illumination device or the like is mounted to the support base 52 at two mutually spaced positions, i.e., a pivotal position at which the mounting member 53 is tiltably mounted to the support base 52 and a tightening position at which the fringes of the arc-shaped groove formed in the disk 53b are tightened by the nut 57 threaded over the threaded shaft 56 of the support base 52. Consequently, the mounting member 53 can be fixed to the support base 52 in a desired tilted state more firmly.
However, it is necessary for the disk 53b itself to hold the posture of the mounting member 53 because the body portion 53a of the mounting member 53 is mounted integrally with the mounting member 53 via the disk 53b. At the same time, the disk 53b and the mounting member 53 having the disk 53b need to be strong enough to support the weight of the device to be held such as an illumination device that might be mounted on the mounting member 53. This increases the size of the mounting member 53 including the disk 53b. 
Furthermore, if the body portion 53a of the mounting member 53 is mounted integrally with the disk 53b and the body portion 53a including the disk 53b is increased in size, the mounting member 53 has a relatively large eccentric heavy portion around the mounting shaft 54 rotatably pivoted to the support base 52. Therefore, at the stage of a work for gripping the support base 52 of the tripod head and mounting the base to a stand or tripod, if the nut 57 is not sufficiently tightened against the threaded rod 56, the heavy mounting member 53 may rotate about the mounting shaft 54 and fall, depending on the posture of the tripod head, even if no device to be held is attached to the mounting member 53.
In this case, falling of the heavy mounting member 53 poses dangers. In addition, a hand is often attached to the mounting member 53 to support it. If the arc-shaped groove 55 is exposed on both sides of the threaded rod 56, fingertips of the hand may enter the groove 55. In this case, there is the possibility that the fingertips in the groove are cut between the fringe of the arc-shaped groove 55 and the nut 57.
Moreover, where a heavy device to be held such as an illumination device is mounted to the mounting member 53 and one attempts to adjust the tilt angle, for example, of the illumination device, the illumination device may rotate about the mounting shaft 54 and fall quickly due to unforeseeable loosening of the nut 57 relative to the threaded rod 56. In this case, too, if fingertips of a hand are inserted in the arc-shaped groove 55, there is the possibility that the fingertips present within the groove are cut off.